1. Field of the Invention
The present invention relates to VLSI chips in general and in particular to the control of electrical noise, electromagnetic interference (EMI) etc., in said chips.
2. Prior Art
Even though the CMOS logic family has several attractive characteristics, its full potential has not been developed because of its reputation of being the noisiest of all the known logic families. As circuit speeds and chip densities increase, it is believed that the noise problems will also increase. This will tend to further limit the use of the CMOS circuit family.
The noise problem is particularly associated with CMOS driver circuits which are sometimes called output buffers. The buffers can be used to drive off chip circuits, on chip internal nets, buses, etc. A conventional CMOS driver or buffer uses two series connected FET devices which should switch sequentially under ideal conditions. However, this ideal condition is never met in practice. Instead, both devices switch simultaneously, thus resulting in a high current transient (di/dt). The high current transient causes ground voltage bounce, noise coupling and radiation problems. A detailed description of ground bounce and other noise associated problems is set forth in the above referenced patent application and documents cited therein. Both the patent application and cited documents are incorporated herein by reference.
The prior art approach for controlling noise is to control the output devices so that only one device conducts at a particular instant of time. Even though this approach works well for its intended purpose, it only applies to single drivers. The prior art technique does not address the noise problem caused by multiple drivers and/or buses.
Another cause of noise in VLSI chips is the simultaneous switching of multiple drivers and/or buses. Because VLSI chips could have multiple buses which could all be switched at the same time, tens or even hundreds of driver circuits could be simultaneously switched to cause objectionable high current transients (di/dt). In fact, this high current transient could be present even though individual drivers are designed to produce non-objectionable current transients for a single signal line.
An example of controlling the outputs from multiple drivers is set forth in U.S. Pat. No. 4,724,340. In this patent, the time prior to a data valid interval is used to set the states of output links to a predetermined value that will insure that no more than half of the I/O drivers will simultaneously switch in the same direction (i.e., On-to-Off or Off-to-On). At best, this approach can only improve the di/dt problem by a factor of 0.5 and may not be acceptable in several situations.